Wire rod having enhanced strength and impact toughness and preparation method for same

ABSTRACT

Provided is a wire rod having enhanced strength and impact toughness. The wire rod includes, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% to 5.0% or less, chromium (Cr): 0.5% to 2.0%, phosphorus (P): 0.020% or less, sulfur (S):0.020% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component, and inevitable impurities. The microstructure of the wire rod includes martensite in an area fraction of 95% or more, and retained austenite (y) as a residual component.

TECHNICAL FIELD

The present disclosure relates to a wire rod having enhanced strength and impact toughness for use in a component of an industrial machine, a vehicle, and the like, exposed to various external load environments, and a method of manufacturing the same.

BACKGROUND ART

Recently, efforts to reduce carbon dioxide emissions, emissions considered one of the main causes of environmental pollution, have become a global issue. In line with this, active movements to regulate vehicle exhaust gas emissions exist. As a measure to comply with such regulations, automakers are attempting to reduce emissions through improvements in fuel efficiency. However, in order to improve fuel efficiency, weight reductions and higher performance are required in vehicles. Thus, the requirement for high strength in automobile materials and components formed thereof has increased. In addition, since demand for resistance to the shock of external impacts has also increased, impact toughness is also recognized as an important material property of an automobile material or an automobile component.

Wire rods having a ferrite or pearlite structure are limited in terms of securing excellent strength and impact toughness therein. Materials having such structures usually have high impact toughness, but have relatively low strength. When cold drawing is performed in order to increase strength, high strength may be obtained. However, there may be a disadvantage in that impact toughness may rapidly decrease in proportion to an increase in strength.

Thus, in order to simultaneously implement excellent strength and impact toughness, a bainite structure or a tempered martensite structure is used. A bainite structure may be obtained through a constant temperature transformation heat treatment using a hot-rolled steel material, while a tempered martensite structure may be obtained through quenching and a tempering heat treatment. However, since the structures described above may not be stably obtained through only using hot rolling and continuous cooling processes according to the related art, additional heat treatment processes described above should be performed using a hot-rolled steel material.

In a case in which high strength and excellent impact toughness is secured without an additional heat treatment, a portion of processes from material production to component production may be omitted or may be simplified, thereby improving productivity and lowering manufacturing costs.

However, since wire rods in which a bainite or martensite structure is stably obtained using hot rolling and continuous cooling processes without additional heat treatment processes have not yet been developed, demand for the development of such a wire rod has emerged.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a wire rod having high strength and excellent impact toughness through only using hot rolling and continuous cooling processes without an additional heat treatment process, as well as a method of manufacturing the same.

Aspects of the present disclosure are not limited to the aspect described above, while other aspects not described above will be clear to those skilled in the art, from the following description.

Technical Solution

According to an aspect of the present disclosure, a wire rod having enhanced strength and impact toughness includes, by wt o, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% to 5.0% or less, chromium (Cr): 0.5% to 2.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and inevitable impurities, and a microstructure includes martensite in an area fraction of 95% or more, and retained austenite (y) as a residual component thereof.

According to another aspect of the present disclosure, a method of manufacturing a wire rod having enhanced strength and impact toughness comprises reheating a steel material including, by wt %, C: 0.05% to 0.15%, Si: 0.2% or less, Mn: more than 3.5% to 5.0% or less, Cr: 0.5% to 2.0%, P: 0.020% or less, S: 0.020% or less, Al: 0.010% to 0.050%, Fe as a residual component thereof, and inevitable impurities; hot-rolling the steel material, having been reheated; cooling the steel material at a rate of 0.2° C./s or higher at a temperature within a range of Mf° C. to Mf-50° C., after the hot rolling; and air cooling the steel material, having been cooled.

Advantageous Effects

According to an aspect of the present disclosure, a wire rod having enhanced strength and impact toughness, required for a material and a component for an industrial machine or a vehicle, may be provided through only using hot rolling and continuous cooling processes.

In addition, an additional heat treatment process according to the related art may be omitted, thereby reducing total manufacturing costs.

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail. The present disclosure is related to a wire rod having excellent impact toughness through only using hot rolling and continuous cooling processes without an additional heat treatment process, such as a constant temperature transformation heat treatment, quenching, and a tempering heat treatment, in order to secure high strength and excellent impact toughness and to a method of manufacturing the same.

First, the wire rod according to the present disclosure will be described in detail. The wire rod according to the present disclosure includes, by wt %, C: 0.05% to 0.15%, Si: 0.2% or less, Mn: more than 3.5% to 5.0% or less, Cr: 0.5% to 2.0%, P: 0.020% or less, S: 0.020% or less, Al: 0.010% to 0.050%, Fe as a residual component thereof, and inevitable impurities.

Hereinafter, a steel component of the wire rod according to the present disclosure and a limitation reason of a composition range will be described in detail (hereinafter, wt %).

C: 0.05% to 0.15%

C is an essential element to ensure strength, and is dissolved in steel or present in a form of carbides or cementite. The simplest method for increasing strength is to form carbides or cementite by increasing a content of C. However, ductility and impact toughness are reduced in this case. Thus, an adjustment of an addition amount of C within a certain range is required. In the present disclosure, the content of C may be added in a range of 0.05% to 0.15%. In a case in which the content of C is less than 0.05%, target strength may be difficult to achieve. In a case in which the content of C exceeds 0.15%, impact toughness may be significantly reduced.

Si: 0.2% or Less

Si is known as a deoxidizing element, along with Al, and is an element for improving strength. Si is known as an element highly effective in increasing strength through solid solution strengthening of a steel material through being dissolved in ferrite, when added to the steel material. However, in a case in which Si is added thereto, strength may be significantly increased, but ductility and impact toughness are significantly reduced. Thus, in a case of a cold forging component requiring sufficient ductility, an addition of Si should be limited. In the present disclosure, in order to secure excellent impact toughness and minimize a decrease in strength, a content of Si of 0.2% or less is included. In a case in which the content of Si exceeds 0.2%, securing target impact toughness may be difficult. In more detail, the content of Si is 0.1% or less.

Mn: More Than 3.5% to 5.0% or Less

Mn allows strength of a steel material to be increased and hardenability thereof to be improved, thereby facilitating formation of low temperature structures, such as bainite or martensite, in a wide range of cooling rates. However, in a case in which a content of Mn is 3.5% or less, hardenability is insufficient. Thus, low temperature structures are difficult to stably secure using a continuous cooling process after hot rolling. In addition, in a case in which the content of Mn exceeds 5.0%, ease of segregation of Mn during solidification of molten metal may be facilitated. In this regard, in the present disclosure, the content of Mn may be in a range of more than 3.5% to 5.0% or less.

Cr: 0.5% to 2.0%

Cr improves strength and hardenability of a steel material in a similar manner to Mn. In detail, Cr improves impact toughness in a case in which Cr is added thereto together with Mn. However, in a case in which the content of Cr is less than 0.5%, strength, hardenability, and impact toughness characteristics are not significantly improved. The content of Cr of more than 2.0% may be effective in improving strength and hardenability, but impact toughness characteristics maybe decreased. In this regard, the content of Cr maybe in a range of 0.5% to 2.0% in the present disclosure.

P: 0.020% or Less

P is a main cause of a reduction in toughness and a reduction in delayed fracture resistance, due to being segregated at a grain boundary, and thus, it is preferable not to include P. Therefore, an upper limit thereof is limited to 0.020% in the present disclosure.

S: 0.020% or Less

S is segregated at a grain boundary, reducing toughness and allowing a low melting point emulsion to be formed so as to inhibit hot rolling. Thus, it is preferable not to include S. Therefore, an upper limit thereof is limited to 0.020% in the present disclosure.

Al: 0.010% to 0.050%

Al is a powerful deoxidizing element and allows oxygen to be removed from steel so as to improve cleanliness. In addition, Al is combined with nitrogen dissolved in steel, forming aluminum nitride (AlN), thereby improving impact toughness. In the present disclosure, Al is actively added. However, in a case in which a content thereof is less than 0.010%, an addition effect thereof is difficult to expect. In a case in which the content thereof exceeds 0.050%, a relatively large amount of alumina inclusions are generated, thereby significantly reducing mechanical properties. In this regard, the content of Al may be in a range of 0.010% to 0.050% in the present disclosure.

In addition to a composition described above, a residual component thereof includes Fe and inevitable impurities. In the present disclosure, an addition of other alloys in addition to an alloy composition described above is not excluded.

Meanwhile, in the present disclosure, contents of Mn, Cr, and C may satisfy Relational Expression 1 below,

4.0≦C(Mn+Cr)⁵/50≦9.0.  [Relational Expression 1]

Here, in Relational Expression 1, Mn, Cr, and C refer to the contents by weight of elements, respectively.

In the present disclosure, a steel material having more excellent impact toughness may be manufactured by controlling the contents of Mn, Cr, and C as illustrated in Relational Expression 1. In other words, Mn and Cr increase hardenability to allow a generation of martensite to be facilitated, even when a cooling rate is relatively low. In addition, Mn and Cr allow low contents of C and Cr to significantly contribute to improvement of impact toughness of martensite.

In addition, in the present disclosure, contents of Mn and Si may satisfy Relational Expression 2 below,

Mn/Si≧22.  [Relational Expression 2]

Here, in Relational Expression 2, Mn and Si refer to the contents by weight of elements, respectively.

In the present disclosure, Mn increases hardenability. Thus, even when a cooling rate is relatively low, Mn allows the generation of martensite to be facilitated. In addition, Si is dissolved in steel, and thus, strength may be increased. However, impact toughness may be reduced.

The inventors have repeatedly conducted research and experiments based on the description above. As a result, the inventors have confirmed that a wire rod having a martensite structure with excellent strength and impact toughness may be provided when the relationship between Mn and Si satisfies Mn/Si≧22 based on weight %, and proposed Relational Expression 2 of a composition component.

Meanwhile, the wire rod according to the present disclosure may be provided with any cross sectional area in which a ratio of a maximum concentration [Mn_(max)] and a minimum concentration [Mn_(min)] of Mn satisfies Relational Expression 3 below,

[Mn _(max) ]/[Mn _(min)]≦4.  [Relational Expression 3]

In the present disclosure, Mn increases hardenability. Even when a cooling rate is relatively low, Mn allows the generation of martensite to be facilitated. However, when Mn is locally segregated, the generation of martensite may be facilitated. Meanwhile, in an area in which Mn is depleted, ferrite may be formed, so that a microstructure may be non-uniform, and impact toughness may be reduced.

The inventors have repeatedly conducted research and experiments based on the description above. As a result, the inventors have confirmed that, when a ratio of a maximum concentration to a minimum concentration of Mn is 4 or less in any cross sectional area of the wire rod, the wire rod having a martensite structure with excellent strength and impact toughness may be provided, and proposed Relational Expression 3.

Hereinafter, a microstructure according to the present disclosure will be described in detail.

The microstructure of the wire rod according to the present disclosure includes martensite of 95 area % or more and residual retained austenite (γ). Martensite according to the present disclosure contains a relatively low content of C. Thus, even though martensite has high strength, ductility and impact toughness thereof is also significantly excellent. However, in a case in which, besides martensite, an amount of bainite or retained austenite is increased, impact toughness may be increased. However, a reduction in strength may not be prevented. Thus, the wire rod according to the present disclosure includes martensite of 95 area % or more.

The wire rod according to the present disclosure may be provided as a material having a circular cross section, tensile strength may be 1000 MPa to 1200 MPa, and an impact value may be 80 J or more.

Subsequently, a method of manufacturing a wire rod according to the present disclosure will be described in detail.

The method of manufacturing a wire rod according to the present disclosure includes: reheating a steel material having a composition described above after the steel material is provided; hot rolling the steel material, having been reheated; cooling the steel material at a rate of 0.2° C./s or higher to a temperature within a range of Mf° C. to Mf-50° C. after the hot rolling; and air cooling the steel material, having been cooled.

First, after a steel material having a composition component described above is provided, the steel material is reheated in the present disclosure. A reheating temperature applied in the present disclosure may be in a range of 1000° C. to 1100° C.

A form of the steel material is not particularly limited, but the steel material may have a bloom or a billet form according to the related art.

Subsequently, the steel material, having been reheated, is hot rolled to manufacture a wire rod. A finish hot rolling temperature of the hot rolling is not particularly limited, but the temperature may be in a range of 850° C. to 950° C.

The steel material having been hot rolled is cooled, and the cooling may be performed at a cooling rate of 0.2° C./s or higher to a temperature within a range of Mf° C. to Mf-50° C. When a cooling end temperature exceeds Mf, a sufficient amount of a martensite structure is difficult to secure. When the cooling end temperature is lower than Mf-50° C., a steel material is sufficiently cooled, so that ease of the handling of the steel material is facilitated. However, since productivity is decreased, the cooling end temperature may be at a temperature within a range of Mf° C. to Mf-50° C. Mf refers to a temperature at which phase transformation from austenite to martensite ends.

In the present disclosure, a martensite structure is secured by performing a continuous cooling process after the hot rolling, thereby securing excellent strength and impact toughness. Here, since a heat treatment, such as quenching and tempering, performed according to the related art, may be omitted, an additional process is not required. Thus, there may be an advantage in terms of manufacturing costs.

In addition, in a section from a cooling start temperature to a cooling end temperature, the cooling may be performed at a cooling rate of 0.2° C./s or higher in the present disclosure. In a case in which the cooling is performed at a cooling rate of 0.2° C./s or higher, and the air cooling is performed, the martensite structure of 95 area % or more may be secured.

MODE FOR INVENTION

Hereinafter, an exemplary embodiment in the present disclosure will be described in detail. The exemplary embodiment described below is provided for the purpose of understanding the present disclosure, and should not be construed as limiting the disclosure thereto.

EXEMPLARY EMBODIMENT

After a molten steel having a composition component of Table 1 below was cast, the cast steel was reheated at 1100° C., the cast steel was wire-rod rolled to have a diameter of 15 mm, the wire-rod was cooled to 150° C., below a temperature, Mf, at a cooling rate of Table 2, and the wire-rod was air cooled, thereby manufacturing a wire rod. Meanwhile, Mf, a martensite phase transformation end temperature, was measured using a dilatometer, slightly varies depending on a chemical composition, and exists in a range of 150° C. to 200° C.

In the wire rod manufactured using a method described above, a microstructure was analyzed, and analysis thereof was illustrated in Table 2. In addition, tensile strength and impact toughness thereof were measured and illustrated in Table 2. Meanwhile, a concentration of Mn was measured using electron probe micro-analysis (SPMA).

In addition, a room temperature tensile test was carried out for measurement, in which a crosshead speed was 0.9 mm/min to a yield point and was 6 mm/min thereafter. In addition, an impact test was carried out at room temperature for measurement, using an impact tester in which curvature of an edge portion of a striker impacting a specimen was 2 mm and test capacity was 500 J.

TABLE 1 Composition component (weight %) Relational Relational No. C Si Mn Cr P S Al Expression 1 Expression 2 1 0.07 0.17 4.1 1.0 0.017 0.020 0.024 4.83 24.1 2 0.09 0.19 3.9 1.4 0.014 0.017 0.029 7.53 20.5 3 0.08 0.15 3.8 0.9 0.011 0.015 0.035 3.67 25.3 4 0.06 0.16 4.7 0.7 0.016 0.013 0.018 5.51 29.4 5 0.12 0.14 3.6 1.5 0.015 0.014 0.034 8.28 25.7 6 0.14 0.20 4.3 1.2 0.011 0.012 0.026 14.09 21.5 7 0.07 0.08 3.7 1.8 0.019 0.013 0.043 7.05 46.2 8 0.11 0.18 4.5 0.8 0.015 0.016 0.015 9.20 25.0 9 0.07 0.16 3.7 2.5 0.014 0.013 0.038 12.83 23.1 10 0.18 0.16 4.2 0.5 0.011 0.015 0.033 8.26 26.3 11 0.11 0.17 5.3 0.8 0.018 0.014 0.027 18.58 31.2 12 0.06 0.15 2.6 1.5 0.016 0.017 0.021 1.39 17.3 13 0.10 0.24 3.8 1.8 0.012 0.011 0.025 11.01 15.8 14 0.08 0.14 3.6 1.4 0.015 0.012 0.032 5.00 25.7 15 0.09 0.18 4.3 0.2 0.017 0.016 0.036 3.32 23.9 (In Table 1, Relational Expression 1 is C (Mn + Cr)⁵/50, Relational Expression 2 is Mn/Si, and a residual component thereof is Fe and inevitable impurities)

TABLE 2 Yield Tensile Cooling rate strength strength Elongation Impact Relational Classification No. (° C./s) (MPa) (MPa) percentage value (J) Expression 3 Inventive 1 6 718 1081 17 96 3.2 Example 2 10 739 1123 16 84 3.1 3 2 721 1105 16 85 3.1 4 20 702 1077 15 96 3.7 5 5 755 1155 16 125 2.9 6 3 756 1171 14 82 3.3 7 0.6 697 1056 19 142 3.0 8 8 720 1080 17 88 3.5 Comparative 9 3 787 1173 12 70 3.1 Example 10 5 815 1232 8 35 3.4 11 2 801 1203 10 41 4.6 12 0.5 514 826 25 120 2.2 13 8 837 1224 8 36 3.0 14 0.05 653 988 13 120 2.9 15 1 721 1125 12 60 3.5 (In Table 2, Relational Expression 3 is [Mn_(max)]/[Mn_(min)].)

As illustrated in Tables 1 and 2, in the cases of Inventive Examples 1 to 8 satisfying a steel composition and a manufacturing method thereof according to the present disclosure, a martensite structure of 95 area % or more may be obtained therefrom. Thus, it can be confirmed that tensile strength of 1000 MPa to 1200 MPa and excellent impact toughness of 80 J or higher are provided.

In the case of Inventive Example 7, a content of Si is 0.1 wt % or less. It can be confirmed that significantly excellent impact toughness and elongation percentage may be secured, compared to other Inventive Examples. Among Inventive Examples, in the cases of Inventive Examples 1, 4, 5, and 7, satisfying Relational Expression 1 (4.0≦C (Mn+Cr) ⁵/50≦9.0) of contents of Mn, Cr, and C, in addition to Relational Expression 2 (Mn/Si≧22.0) of Mn and Si, it can be confirmed that impact toughness is further excellent, compared to different cases.

In other words, among Inventive Examples, in the cases of Inventive Examples 2, 3, 6, and 8, not satisfying Relational Expression 1 (4.0≦C(Mn+Cr)⁵/_(50≦9.0)) and/or Relational Expression 2 (Mn/Si≧22.0), it can be confirmed that impact toughness is somewhat reduced.

Comparative Example 9 is a case in which a component of Cr is outside of a range of the present disclosure and illustrates that toughness is increased, but ductility is decreased, so that impact toughness is reduced. Comparative Example 10 is a case in which a content of C exceeds the range of the present disclosure and has a problem in which toughness is significantly increased, due to a solid solution strengthening improvement effect of a martensite base of C, but impact toughness is significantly reduced.

Comparative Example 11 is a case in which a component of Mn is outside of the range of the present disclosure and illustrates that toughness is increased, but ductility is decreased, so that impact toughness is reduced. In addition, it can be confirmed that Mn is segregated in steel, and a locally uneven structure is formed, so that impact toughness may be reduced.

Comparative Example 12 is a case in which Mn is added in an amount less than a composition range of the present disclosure. Since hardenability is relatively low, a bainite structure, rather than a martensite structure, is formed when a cooling rate is relatively low. Thus, it is confirmed that impact toughness is increased, but strength is reduced. In addition, Comparative Example 13 is a case in which Si is contained in an amount exceeding the composition range of the present disclosure. It can be confirmed that, when an addition amount thereof is 0.52%, tensile strength is significantly increased, while impact toughness is significantly reduced.

Comparative Example 14 satisfies a composition component of steel of the present disclosure. However, when a cooling rate is significantly low, the bainite structure, rather than the martensite structure, is formed. Thus, it is confirmed that impact toughness is increased, but strength is reduced. Furthermore, it can confirmed that Comparative Example 15 containing a relatively small amount of Cr has relatively poor impact toughness.

While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims. 

1. A wire rod having enhanced strength and impact toughness comprising, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% to 5.0% or less, chromium (Cr): 0.5% to 2.0%, phosphorus (P): 0.020% or less, sulfur (S):0.020% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component, and inevitable impurities, wherein a microstructure includes martensite in an area fraction of 95% or more, and retained austenite (y) as a residual component.
 2. The wire rod having enhanced strength and impact toughness of claim 1, wherein contents of Mn, Cr, and C satisfy Relational Expression 1 below, 4.0≦C(Mn+Cr)⁵/50≦9.0.  [Relational Expression 1]
 3. The wire rod having enhanced strength and impact toughness of claim 1, wherein contents of Mn and Si satisfy Relational Expression 2 below, Mn/Si≧22.  [Relational Expression 2]
 4. The wire rod having enhanced strength and impact toughness of claim 1, wherein the wire rod is provided with any cross section in which a ratio of a maximum concentration [Mn_(max)] to a minimum concentration [Mn_(min)] of Mn satisfies Relational Expression 3 below, [Mn _(max) ]/[Mn _(min)]≦4.  [Relational Expression 3]
 5. A method of manufacturing a wire rod having enhanced strength and impact toughness, comprising: reheating a steel material including, by wt %, C: 0.05% to 0.15%, Si: 0.2% or less, Mn: more than 3.5% to 5.0% or less, Cr: 0.5% to 2.0%, P: 0.020% or less, S: 0.020% or less, Al: 0.010% to 0.050%, Fe as a residual component, and inevitable impurities; hot rolling the steel material, having been reheated; cooling the steel material at a rate of 0.2° C./s or higher at a temperature within a range of Mf° C. to Mf-50° C., after the hot rolling; and air cooling the steel material, having been cooled.
 6. The method of claim 5, wherein contents of Mn, Cr, and C satisfy Relational Expression 1 below, 4.0≦C(Mn+Cr)⁵/50≦9.0.  [Relational Expression 1]
 7. The method of claim 5, wherein contents of Mn and Si satisfy Relational Expression 2 below, Mn/Si≧22.  [Relational Expression 2]
 8. The method of claim 5, wherein the reheating is performed at a temperature of 1000° C. to 1100° C.
 9. The method of claim 5, wherein finish hot rolling of the hot rolling is performed at a temperature within a range of 850° C. to 950° C. 